The invention relates generally to techniques for drilling relatively large-diameter shafts for use as structural foundation piles, and more particularly to core barrels for constructing piling for buildings, bridges and the like.
In the foundation drilling industry, it is desired to drill relatively large diameter shafts (on the order of 36 inches to 48 inches and up) in the earth, and these shafts are typically filled with reinforced concrete to form foundation piles for buildings, bridges, etc. Often a complete shaft is drilled, such as by auguring. Alternatively, in the so-called drill shaft construction technique, a large diameter, hollow core barrel is rotated so that cutters on its lower edge cut an annular kerf in the ground, which is typically rock or rocky ground. Once this kerf is drilled to the desired depth by the core barrel""s cutting face, the rock core within the kerf may be broken up and augured out, or broken off and removed to permit the shaft to be filled with reinforced concrete for forming a pile. Alternatively, the core may be left in place, with the pile being formed by filling the annular kerf with cementitious material, steel casement, or other suitable means for forming the outermost portion of the pile. An example of the latter technique is disclosed in my U.S. Pat. No. 5,823,276, the contents of which are incorporated herein by reference.
The use of sharp cutters to score the rock and form removable cuttings is but one way to drill shafts. In the prior art, there are also rotating, double-walled core barrels that have roller bits as the cutting surface. These roller bits are typically welded to the bottom of the barrel. As the core barrel rotates and the cutters scrape cuttings from the bottom of the kerf, pressurized air is circulated down between the double walls via a swivel through the rotary, thereby flushing cuttings up past the outer diameter of the core barrel and out of the kerf. The foregoing cutting techniques generally require extreme downward pressure on the core barrel.
For applications which only require smaller-diameter shafts, such as oil and gas drilling, it is known to use pneumatic, percussive-type downhole drills, which permit significant reductions in the amount of downward pressure that must be applied to the drilling apparatus. These small downhole xe2x80x9chammerxe2x80x9d drills typically employ a drill bit with a circular cutting face having numerous protruding tungsten carbide buttons. A rotary head or kelly-bar drive causes the drill string to rotate in the shaft, and drilling pipes conduct compressed air to a piston (i.e., the hammer) near the end of the drill string, generating percussive blows of the cutting face of the drill bit to the earth at the bottom of the shaft. These percussive blows place the rock in compression, and the retreating drill bit places the rock in tension. This cyclic action, which may occur several hundred times per minute, breaks up the rock, which is then removed by a drilling fluid (often, simply air) which is circulated down into the shaft under pressure. Rotation of the drill string brings the drill bit into contact with fresh unbroken rock during successive percussion cycles.
Single downhole drills of the type described are typically from a few inches up to about 34 inches in diameter. Greater diameters are impractical due to the excessive cost of larger-diameter drill bits and large downhole hammers. To achieve larger-diameter shafts, it is known to use cluster drills comprising a plurality of hammer drills in a gang construction, as described in U.S. Pat. No. 4,729,439 to Kurt. In gang drills of this type, several hammer drills are arranged within a casing in a ring around a central hammer drill which is concentric with the casing and thus the shaft to be drilled. The cutting faces of the drill bits must be sufficiently large to cut swaths which completely cover the bottom of the shaft. For relatively large diameter shafts, e.g., 36 inches and greater, the number and size of hammer drills required makes their use impractical because air and fuel consumption tends to be quite high. In addition, noise levels for gang drills of this size (and large-diameter single downhole drills) may be intolerable, particularly if drilling is to occur near populated areas. Also, gang drills of this size suffer from disadvantages such as excessive weight and cost, and limited ability to be manhandled.
What is needed is a drilling apparatus which makes use of hammer-type drills and is suitable for drilling large-diameter shafts but does not suffer from the disadvantages of excessive air and fuel consumption, cost, weight and noise which accompany conventional gang drills and large-diameter single downhole drills.
Accordingly, an object of the present invention is to provide an improved hammer-type drill suitable for large-diameter applications having lower air and fuel consumption than conventional large-diameter gang drills.
Another object of the present invention is to provide an improved hammer-type drill suitable for large-diameter applications having lower cost and weight, and higher maneuverability, than conventional large-diameter gang drills and large-diameter single downhole drills.
Another object of the present invention is to provide an improved hammer-type drill suitable for large-diameter applications which does not require that drill bits cut a swath across the entire bottom of the shaft.
A further object of the present invention is to provide a core barrel for drilling foundation piles and the like without the use of cutters requiring large downward pressures on the barrel.
In satisfaction of these and other objects, the invention provides a hollow core barrel with a plurality of hammer drills disposed around its circumference at its working end. The core barrel is preferably double-walled, with the plurality of hammer drills disposed within the walls and positioned vertically so that the drill bits extend beyond the bottom end of the barrel. A pressurized air source is coupled to the core barrel at its top end. A manifold arrangement conducts air from the air source to conduits within the walls of the core barrel, which in turn deliver pressurized air to each hammer drill.
The core barrel has a diameter suitable for drilling foundation piles for buildings, bridges and the like. The diameter of the core barrel is typically 36-48 inches, although diameters of 72 inches or more may be realized. In practice, the diameter will be at least about 18 inches to produce piles suitable for use in foundations and related systems.
In operation, the core barrel is rotated by a top drive rotary or kelly bar on a conventional drilling rig. The core barrel has an opening at its top end for admitting a drilling fluid, or slurry, delivered thereto via a conduit connected to a swivel means located above the top drive rotary. As the core barrel is rotated to cut an annular kerf, the slurry is pumped downward between the double walls of the core barrel, toward the hammer drill bits. The slurry flows across the bit faces, simultaneously cooling the bits and washing the bottom of the kerf of particles dislodged by the bits. The slurry, laden with cuttings, then exits the kerf upward between the outside diameter of the core barrel and the excavated wall of the shaft, as well as between the inside diameter of the core barrel and the rock core.
Air from the pressurized air source is also exhausted from the shaft, carrying cuttings out of the kerf. Like the slurry, air is exhausted upward between the outside diameter of the core barrel and the excavated wall of the shaft. The top of the core barrel is provided with a plurality of vents to allow slurry and exhausted air to escape from the interior of the core barrel, thereby preventing excessive pressure build-up under the core barrel, which could otherwise impede drilling progress or even drive the core barrel upward out of the shaft. Exhaustion of the pressurized air is also accomplished by a plurality of conduits placed between the walls of the core barrel and extending its entire length to openings in the top surface of the core barrel.
Once the kerf is drilled to the desired depth, the core barrel is withdrawn from the kerf, and the core may be removed by any conventional technique. The resultant excavation may be cleaned, and then filled with cementitious material (such as concrete) and reinforcing steel to complete the pile. Alternatively, the core may be left in place to form the interior portion of a structural pile. In such a construction, the drilled annular kerf is filled with cementitious material, or a combination of cementitious material and reinforcing steel. Alternatively, a full-length metallic casing, such as steel, may be placed in the annular kerf. If necessary, the resulting annular spaces on both sides of the shell casing may then be grouted with cementitious material, sand or the like.
A hammer-type core barrel according to the present invention does not require excessive downward pressure to cut the kerf, unlike many conventional core barrels. In addition, it has numerous advantages over large-diameter gang drills. Since only a relatively narrow kerf is cut, rather than the entire interior of the shaft, drilling proceeds much more rapidly. Smaller and lighter hammer drills can be used, with attendant lower weight and cost, lesser requirements for air and fuel, and increased ease of handling and serviceability. Drilling noise may be substantially reduced, as well as pollution from airborne dust and earthen particles which are flushed from the shaft.